In my youth, I had summer jobs at the Post Office, learning how parcels are handled. The general public grossly underestimates the strain a parcel must be able to withstand during transportation. (As illlustrated by #35-#40!)

Another aspect: Standardizing on a few box sizes allow the boxes to be packed densely during transportation. It is better to have the air inside each box (enclosed in some sort of bubble wrapper) than between the boxes, which would otherwise be the case.

Of course there are limits to what is reasonable packing, and sometimes that limit is exceeded. But weigt is a much more important cost factor than volume; volume is CHEAP. If you can buy extra protection and ease of handling by rounding up the volume to a few standard sizes, it will often pay back many times the cost of the "wasted" volume.